The flow rate sensor disclosed in patent document 1 represents a conventional art of a flow rate sensor. The flow rate sensor disclosed in patent document 1 is so arranged that two resistive elements whose resistance value changes in accordance with a temperature of a fluid are arranged mutually independently in an upstream side and a downstream side of a channel where the fluid flows, and two constant temperature control circuits, each of which contains the resistive element, respectively, are arranged mutually independently so that a temperature of the resistive element is continually controlled to be constant and equal by the constant temperature control circuits, and calculates a flow rate (Q) based on an expression, (Vu−Vd)/(Vu+Vd), where an output of, the constant temperature control circuit corresponding to the upstream resistive element is defined as Vu, and an output of the constant temperature control circuit corresponding to the downstream resistive element is defined as Vd.
The reason why the above-mentioned expression is used is because it is considered that Vu+Vd is an amount influenced by the ambient temperature alone and an increasing rate (a gradient) of the output of the flow rate sensor relative to a change of the actual flow rate (Q) can be made at a constant value, irrespective of the ambient temperatures, by conducting division by Vu+Vd so that the linearity of the output of the flow rate sensor can be improved and the error can be made further smaller.
However, since practically the absolute value of the gradient of the output Vu of the constant temperature control circuit relative to the flow rate (Q) is different from the absolute value of the gradient of the output Vd of the constant temperature control circuit relative to the flow rate (Q) as shown in FIG. 5, Vu+Vd does not take a constant value relative to the flow rate (Q) and Vu+Vd is a value that varies in accordance with the flow rate (Q).
As a result of this, although it is possible for the above-mentioned calculation to lessen an influence from the ambient temperature on the output of the flow rate sensor, another non-linearity is added by the influence from the change of the flow rate (Q) because the output is divided by Vu+Vd so that the linearity of the output of the flow rate sensor can not be improved and the error can not be smaller.
In order to solve these problems, patent document 2 suggests that the above-mentioned expression be changed to (Vu−Vd)/(αVu+βVd), where (0≦α, β≦1, α≦β) so that the influence from the change of the flow rate (Q) is reduced and the linearity of the output of the flow rate sensor is improved.
However, even though the flow rate sensor described in patent document 2 is arranged so that the increasing rate (the gradient) of the output of the flow rate sensor relative to the change of the actual flow rate (Q) is difficult to change relative to the ambient temperature, the error still remains because the increasing rate is influenced by the change of the flow rate (Q).
In addition, neither patent document 1 nor patent document 2 considers how to correct the zero point output in a state where the flow rate (Q) is zero. As a result of this, since the shift amount of the sensor output changes largely relative to the ambient temperature change, the change of the shift amount is considered to be a part of the reason for an error in the output of the flow rate sensor.